feat(api): BA-117 add bounded request body size and reject oversized JSON payloads

[ranjeetprabhatedu-dotcom]

Summary

This PR implements BA-117 to add a bounded request body size, preventing denial-of-service and memory exhaustion via oversized payloads.

• Configures express json({ limit }) and urlencoded({ limit }) body parsers in main.ts with configurable MAX_BODY_SIZE (defaulting safely to 1mb).
• Exports REQUEST_BODY_LIMIT provider in AppModule so the limit is recognized across all application contexts.
• Adds comprehensive unit tests in app.controller.spec.ts verifying that payloads exceeding the configured body threshold are rejected with HTTP 413 entity.too.large, while normal valid payloads parse successfully.
• Scoped strictly to the 3 allowlisted files:
  • BackendAcademy/src/main.ts
  • BackendAcademy/src/app.module.ts
  • BackendAcademy/src/app.controller.spec.ts

Closes #1030